There has been a conventionally known head-up display apparatus (hereafter, referred to as HUD apparatus) installed in a movable body (hereafter, referred to as vehicle), such as an automobile. The HUD apparatus is so constructed that the following is implemented: varied information is projected as an image light from an image projector disposed inside the instrument panel of the vehicle; and it is reflected by the inside surface of a windshield so that the driver can visually perceive the varied information as a virtual image (hereafter, referred to as display image).
In general, such an HUD apparatus is so constructed that a display image is displayed in superimposition on the view in front of the vehicle. For the driver to be capable of easily focusing his/her eyes on a display image to visually perceive it, it is important to take the following measure: the display image is projected to a position closest to the front view as much as possible (a position distant from the position of the driver's eyes). For this purpose, it is required to lengthen the distance between the driver who is to visually perceive a display image and the image projector. However, this involves a problem. When the distance between the driver who is to visually perceive a display image and the image projector is simply lengthened, the size of the HUD apparatus itself is increased.
To cope with this, for example, the following measures are taken to shorten the overall length of a HUD apparatus to reduce its size: multiple reflecting mirrors are installed in the optical path between an image projector and a windshield to bend the optical path; a magnifying lens is disposed between an image projector and a windshield to equivalently lengthen the optical path; and the like. (Refer to Patent Document 1, for example.)
FIG. 21 explains the principle on which an optical path can be equivalently lengthened by a common convex lens as a magnifying lens. When an image projector for projecting an image light 100 is disposed between the focal point F of the convex lens C and the convex lens C, the following takes place as illustrated in FIG. 21: the obtained display image 101 is a virtual image and is displayed (i.e., the image is formed) in the same direction as the direction in which the image projector is disposed as viewed from the convex lens C.
It will be assumed that: the distance between the center of the convex lens C and the display image 101 visually perceived by the driver should be b; the distance between the center of the convex lens C and the part of the image projector for projecting the image light 100 be a; and the focal length of the convex lens C be f. Then, the relation expressed by Expression (1) holds between the distance a, distance b, and focal length f.
                    [                  Expression          ⁢                                          ⁢          1                ]                                                                                  1            a                    -                      1            b                          =                  1          f                                    (        1        )            
More specific description will be given. As is apparent from Expression (1), the following can be implemented by lengthening the distance a between the image projector for projecting the image light 100 and the convex lens C within the range from the convex lens C to the focal point F: the distance b to the projection position of the display image can be lengthened, that is, the optical path from the image projector to the driver who is to visually perceive the virtual image can be equivalently lengthened.
In the HUD apparatus, however, the windshield 203 exists between the lens 202 (convex lens C) and the driver and image projector is so disposed that an image light is projected to the windshield 203 from beneath, as illustrated in FIG. 22. That is, the driver visually perceives the image light 100 whose traveling direction has been changed by being reflected by the windshield 203. As a result, the virtual image appears in a position beyond the windshield 203, not under the windshield 203 under which the image projector is positioned.    Patent Document 1: JP 2002-202475 A (corresponding to US 2002/0084950 A1)
However, the HUD apparatus described in Patent Document 1 involves a problem. Depending on the angle at which outside light, such as sunlight 200, comes in, it passes through the windshield 203 and is reflected by the lens 202 and the reflected light 201 can enter the driver's eye range 204 as illustrated in FIG. 23.
As a result, the problem illustrated in FIG. 24 arises. That is, the reflected light 201 is more intense than the light of the display image; therefore, it becomes so intense noise that it is impermissible for drivers and affects the visibility of a display image.